1. Field of the Invention
The invention relates to producing linear alpha olefins from synthesis gas over a cobalt catalyst. More particularly the invention relates to producing C4-C20 linear alpha olefins having low amounts of oxygenates, by reacting H2 and CO in a synthesis gas produced from natural gas, over a non-shifting cobalt catalyst at reaction conditions of temperature, % CO conversion, H2:CO mole ratio and water vapor pressure effective for the mathematical expression of 200xe2x88x920.6T+0.03PH2Oxe2x88x920.6XCOxe2x88x928(H2:CO) to have a numerical value greater than or equal to 50. This can be integrated into a Fischer-Tropsch hydrocarbon synthesis process producing fuels and lubricant oils.
2. Background of the Invention
Linear alpha olefins in the C4-C20 carbon atom range are large volume raw materials used in the production of, for example, polymers, detergents, lubricants and PVC plasticizers. The demand for these olefins is rapidly increasing, particularly for those that have from 6 to 12 carbon atoms, such as six and eight carbon atom linear alpha olefins desirable for making polyolefin plastics Most linear alpha olefins are produced by ethene oligomerization, for which the ethene feed cost can account for more than half the total cost of the alpha olefin production. It is known that alpha olefins can be produced from synthesis gas using iron, iron-cobalt, iron-cobalt spinel, copper-promoted cobalt and cobalt manganese spinel catalysts, most of which are shifting catalysts. Examples of linear alpha olefin production with such catalysts may be found, for example, in U.S. Pat. Nos. 4,544,674; 5,100,856; 5,118,715; 5,248,701; and 6,479,557. U.S. Pat. No. 6,479,557, for example, discloses a two-stage process to make paraffinic hydrocarbons in the first stage and olefinic hydrocarbons in the second stage. The paraffinic product is made by converting a substoichiometric synthesis gas feed (i.e., H2/CO feed ratio lower than about 2.1:1) over a non-shining catalyst in the first stage. Since the H2/CO usage ratio is stoichiometric, the effluent of the first stage is significantly depleted in CO. This effluent of the first stage is then used to make olefinic hydrocarbons in the second stage over a shifting Fischer-Tropsch catalyst.
Although iron-based shifting catalysts produce hydrocarbons from synthesis gas with high alpha olefin content even at high CO conversion levels, the undesirable water gas shift reaction associated with shifting catalysts wastes part (as much as 50%) of the CO feed by converting CO to CO2. Furthermore, in addition to high CO loss due to the water gas shift conversion of CO to CO2, iron-based catalysts produce linear alpha olefins containing more than 1 and even as much as 10 wt. % oxygenates. These oxygenates are poisons to catalysts used for producing polymers and lubricants from olefins. Hence, the concentration of oxygenates must be reduced to a level acceptable for polymer and lubricant production. The methods used for removing the oxygenates are costly, thus catalysts and processes yielding olefin products with low oxygenate content is highly desired.
It would be an improvement to the art if a way could be found to (i) produce linear alpha olefins with low oxygenates levels and particularly with (ii) non-shifting catalyst and preferably a non-shifting Fischer-Tropsch hydrocarbon synthesis catalyst that is also useful for synthesizing fuel and lubricant oil fractions. It would be a still further improvement if (a) linear alpha olefin production could be integrated into a Fischer-Tropsch hydrocarbon synthesis process and (b) if a hydrocarbon synthesis reactor employing a non-shifting Fischer-Tropsch hydrocarbon synthesis catalyst and producing fuel and lubricant oil fraction hydrocarbons could also be used for linear alpha olefin production, and vice-versa, without having to change the catalyst in the reactor.
The invention relates to a process for producing linear alpha olefins, and particularly linear alpha olefins having from four to twenty carbon atoms, having less than 3 and preferably less than 1 wt. % oxygenates, by reacting H2 and CO, in the presence of a non-shifting Fischer-Tropsch hydrocarbon synthesis catalyst comprising a catalytic cobalt component, under reaction conditions defined by a Condition Factor (CF) greater than or equal to 50, which Condition Factor is defined as:
CF=200xe2x88x920.6T+0.03PH2Oxe2x88x920.6XCOxe2x88x928(H2:CO)
where,
T=average reactor temperature in xc2x0 C.; the average reactor temperature is calculated by averaging the temperature readings from thermocouples measuring the temperatures of individual segments of the reactor. For example, if the temperature is measured in the middle of the first, second, and third equal-volume segments of a fixed bed reactor, the average temperature is equal to one third of the sum of the three readings.
PH2O=partial pressure of the water in the synthesis gas feed to the reactor, in kPa; the partial pressure of water in the feed is calculated by multiplying the mol fraction of water in the feed by the feed pressure measured in kPa. The mol fraction of feed components can be determined by, for example, using gas chromatographic methods.
XCO=CO conversion expressed as percent; the CO conversion can be determined from the CO balance. There are many methods available for establishing material balance. The method herein utilized measurements based on an internal standard such as a noble gas or nitrogen that is inert during Fischer-Tropsch synthesis. When using an inert internal standard, the conversion can be simply calculated by measuring the concentrations of CO and the internal standard in the feed and the effluent. This and other calculation methods are well known in the art of chemical engineering. The concentrations of CO and the inert internal standard in turn can be determined by gas chromatographic methods known in the art.
H2:CO=H2 to CO molar ratio in the synthesis gas feed to the reactor; the concentrations of H2 and CO in the feed can be determined by gas chromatographic methods.
By nonshifting is meant that under the reaction conditions the catalyst will convert less than 5 and preferably less than 1 mole % of the CO to CO2 up to 90% CO conversion in Fischer-Tropsch synthesis. The wt. % of oxygenates is meant the wt. % of oxygenates in the total synthesized C4-C20 hydrocarbon fraction, and by oxygenates is meant oxygen-containing hydrocarbon molecules, such as alcohols, aldehydes, acids, esters, ketones, and ethers. The process of the invention has been found to produce a C4-C20 hydrocarbon fraction containing more than 50 wt. % linear alpha olefins and less than 3 wt. % preferably less than 1 wt. % oxygenates. This process can be achieved as a stand-alone process or it can be added to or integrated into a Fischer-Tropsch hydrocarbon synthesis process. The relatively low selectivity for alpha olefin production normally exhibited by a non-shifting Fischer-Tropsch cobalt catalyst, is at least partially overcome by operating the hydrocarbon synthesis reactor under reaction conditions in which the CF, according to the above expression, is greater than or equal to 50. By CO conversion is meant the amount of CO in the synthesis gas feed converted in a single pass through the reactor.
In another embodiment the invention relates to (a) producing a CO and H2 containing synthesis gas from natural gas, (b) reacting the H2 and CO containing synthesis gas in the presence of a non-shifting cobalt Fischer-Tropsch hydrocarbon synthesis catalyst, at reaction conditions effective to achieve a Condition Factor (CF) greater than or equal to 50, to synthesize linear alpha olefins, and particularly linear alpha olefins having from four to twenty carbon atoms having less than 3 wt. %, preferably less than 1 wt. % oxygenates. A process in which natural gas is converted to synthesis gas which, in turn, is converted to hydrocarbons, is referred to as a gas conversion process. In yet another embodiment, the process of the invention relates to an integrated gas conversion process, in which the linear alpha olefin production process of the invention is integrated with a hydrocarbon synthesis process which produces primarily fuel and lubricating oil products. This is explained in detail below.
It is preferred in the practice of the invention that the synthesis gas be produced from natural gas. Natural gas typically comprises mostly methane, for which the H:C ratio is 4:1 and is therefore an ideal feed for producing synthesis gas having a nominal H2:CO mole ratio of 2:1 or somewhat higher, for example 2.1:1. Substantial amounts of hydrogen can be separated from a synthesis gas with a H2:CO=2:1 mole ratio, to produce H2 and a 1:1 H2:CO mole ratio synthesis gas. The 1:1 H2:CO mole ratio is a preferred ratio for the linear alpha olefin process of the invention. Thus, while the synthesis gas produced in the synthesis gas generating reactor of a gas conversion plant typically has an H2:CO mole ratio of 2.1:1 or 2:1, all or a portion of this synthesis gas may be optionally treated to change the H2:CO mole ratio in the gas to a more preferred ratio for the alpha olefin synthesis process, before it is passed into the one or more linear alpha olefin producing reactors.
It is understood that although the chemistry involved in producing linear,alpha olefins over a non-shifting Fischer-Tropsch cobalt catalyst is such that it is preferred for the H2:CO mole ratio in the synthesis gas feed passed into the linear alpha olefin reactor be typically less than 2:1, the stoichiometric H2:CO consumption mole ratio of the linear alpha olefin reaction is 2:1. Furthermore, it is also understood that conventional hydrocarbon synthesis making paraffinic hydrocarbons for fuel and lubricant applications over a non-shifting cobalt catalyst employs a synthesis gas feed in which the H2:CO mole ratio is 2.1:1. In an integrated gas conversion process of the invention, one or more reactors may be added to and/or switched back and forth from hydrocarbon synthesis for producing fuel and lubricant fractions to linear alpha olefin production by changing the conditions from the conventional hydrocarbon synthesis conditions to the alpha olefin selective conditions of the present invention by adjusting the reaction parameters to achieve a CF value of greater than or equal to 50 and vice versa. Thus, producing the synthesis gas from natural gas provides a particular synergy and flexibility for practicing all embodiments of the present invention.
A CF value of greater than or equal to 50 can be achieved by many different combinations of temperature, CO conversion, H2:CO ratio and water partial pressure. The individual process conditions that are preferred for achieving CF greater than or equal to 50, and thus high alpha olefin selectivity and concomitant production, include (a) setting the H2:CO ratio in the synthesis gas feed to a value less than 2.1:1 and preferably less than 1.8:1, (b) a CO feed conversion of less than 50 and preferably less than 30% in a single pass through the reactor, and (c) a reaction temperature typically between 160 and 250 and preferably between 180 and 240xc2x0 C. The presence of water in the synthesis gas feed, while optional, is preferred. Thus, it is possible for the value for PH2O in the expression above to be zero or negligible but a non-zero value in the range of from 50 to 500 kPa is preferred.
It is understood that although the above-specified preferred ranges provide guidance for the typical values of the individual control variables, (i.e., average reactor temperature, partial pressure of water and H2:CO ratio in the gas feed to the reactor, and CO conversion) under which a Condition Factor value of greater than or equal to 50 can be achieved, it is the combination of the control variables which satisfy the CF described herein which is the invention not the individual variables alone . Thus the skilled artisan should appreciate that the invention is a linear combination of the control variables as defined by the Condition Factor. The utility of the Condition Factor is that it enables the determination of this preferred combination of the control variables. Thus, for example, if for economic or process reasons the feed to the reactor does not have steam (i.e., PH2O=0 kPa), and the CO conversion needs to be at least 30%, the H2:CO ratio in the feed to the linear alpha olefin reactor and the average reactor temperature need to be set so that the sum of 0.6(T) and 8(H2:CO) is less than 200xe2x88x9218xe2x88x9250=132. If, again, for process reasons the temperature is set for 205xc2x0 C., the H2:CO ratio needs to be less than 1.125:1 for the process of the invention. Clearly, if three control variables are set for process or economic reasons, the preferred value range of the fourth variable can be readily calculated. Those skilled in the art will also recognize, that if only two control variables are fixed, the preferred combinations of the remaining two variables will define a two-dimensional surface which will be further defined by some other common boundary conditions known in the art, like that the H2:CO ratio cannot be equal or less than zero (no hydrocarbons form in the absence of H2) or that the reaction temperature cannot be below 160xc2x0 C. (where for the catalyst systems of interest to the process of this invention, the Fischer Tropsch reaction rate is too low to be practical). Likewise, if only one control variable is fixed, the preferred conditions define a three-dimensional space. It is clear, therefore, that while the specific preferred ranges given earlier for the individual control variables provide reasonable starting points, the ultimate combination of the control variables of the invention need to be derived from the expression provided herein for the Condition Factor.
Thus, although Applicants have specified preferred ranges for the input variables for CF (T, PH2O, XCO and H2:CO) so long as the specified CF is met, the input variables may vary from the ranges specified herein. Hence the CF criteria disclosed herein, is the criteria which must be met when setting the noted input variables.
Lower CO conversion can be readily achieved by increased, synthesis gas feed rates through the reactor, which also results in shorter product residence times. As a consequence, synthesis gas feed rate is another variable that may be used to achieve the desired conversion level and thus a CF greater than or equal to 50. Thus, the synthesis gas feed rate (commonly quantified as Gas Hourly Space Velocity or GHSV) through the reactor, with a non-shifting catalyst such as disclosed in U.S. Pat. No. 5,945,459, U.S. Pat. No. 5,968,991, U.S. Pat. No. 6,090,742, U.S. Pat. No. 6,136,868, U.S. Pat. No. 6,319,960, RE 37,406, U.S. Pat. No. 6,355,593, U.S. Pat. No. 6,331,575 comprising a catalytic cobalt component, will typically be greater than 15,000 standard volumes of gas (measured at 103 kPa and 25xc2x0 C.)/volume of catalyst/hour (V/V/hr), and preferably greater than 25,000 V/V/hr. It is understood, however, that the feed rate necessary to maintain the preferred CO conversion of the invention will also depend on the volumetric productivity of the catalyst. Hence, as would be readily apparent to the skilled artisan, a catalyst that has two times higher volumetric activity, will require two times faster feed rate to maintain the same CO conversion or 50,000 v/v/hr.
These conditions, particularly the low CO feed conversion and high synthesis feed gas rate through the reactor, are more readily achieved in a reactor containing one or more fixed beds of catalyst or a fluidized catalyst. A slurry reactor, highly efficient for synthesizing higher molecular weight paraffinic hydrocarbons, may also be utilized for the olefin synthesis, provided that the appropriate residence times are maintained. Such residence times are readily determined by the skilled artisan due to the inherently longer product residence time.
In order to maximize the hydrocarbon product yield in the C4-C20 carbon range, particularly the linear alpha olefin yield in the C4-C20 carbon range; the hydrocarbon synthesis reaction is preferably conducted at an alpha of less than 0.9 and more preferably less than 0.8. This is in contrast to an alpha of at least, and preferably greater than 0.9, which is desirable for synthesizing higher molecular weight hydrocarbons for fuel and lubricant applications.
In a broad embodiment, the invention relates to a process for synthesizing C4-C20 linear alpha olefins, wherein the process comprises passing a synthesis gas feed comprising a mixture of H2 and CO through a linear alpha olefin hydrocarbon synthesis reactor, in which it contacts a non-shifting Fischer-Tropsch hydrocarbon synthesis catalyst comprising a catalytic cobalt component, at reaction conditions sufficient for the H2 and CO to react and form the linear alpha olefins and wherein the reaction conditions are such that the following expression has a value greater than or equal to 50:
200xe2x88x920.6(T)+0.03PH2Oxe2x88x920.6XCOxe2x88x928(H2:CO),
and wherein,
T=average reactor temperature in xc2x0 C.
PH2O=partial pressure of the water in the synthesis gas feed to the reactor, in kPa
XCO=CO conversion, expressed as percent, and
H2:CO=hydrogen to CO molar ratio in the synthesis gas feed to the reactor.
The above expression defines the Condition Factor (CF). Thus, the preferred reaction conditions of the process of the invention can also be described as a combination of average reaction temperature, water partial pressure in the feed, CO conversion, and feed H2:CO ratio that yields a CF value of greater than or equal to 50.